1. Field of the Invention
The present invention relates generally to polarization splitters, combiners and isolators, and particularly to a wavelength-insensitive apparatus for splitting or combining a randomly polarized light signal.
2. Technical Background
Polarization is a critical parameter in optical communication technology. In a single mode fiber, the fundamental mode of the fiber is the solution to the wave equation that satisfies the boundary conditions at the core-cladding interface. However counterintuitive this may seem, there are two solutions to the wave equation that correspond to the fundamental mode. The fiber is deemed a single mode fiber because both solutions have the same propagation constant, at least in a perfectly cylindrical fiber. The two solutions are referred to as the polarization modes. The electric field associated with the fundamental mode is assumed to be a transverse field, with the polarization components being linearly polarized along the x and y directions. Thus, the polarization components are mutually orthogonal. As light travels down the fiber, the energy of the pulse is divided between the two polarization modes. The state of polarization refers to the distribution of light energy between the two polarization modes. In practice, since fibers are not perfectly circular, the two polarization modes have slightly different propagation constants that give rise to pulse spreading. This phenomenon is called polarization mode dispersion.
The polarization state of light travelling in a fiber optic network must be taken into account during the design. Optical fiber can be made polarization independent with respect to polarization mode dispersion, but the state of polarization can vary over all states, with respect to time, and be affected by environmental factors. A number of devices require incident light signals to be in a particular polarization state. The performance of such devices will change significantly with the state of incoming polarization. Thus, when the incident light signal is randomly polarized, the device will not function.
One approach that has been considered involves the use of polarization maintaining (PM) fibers. While PM fiber will maintain the polarization state of the light signal, it is not practical for most communications systems for several reasons. First, attenuation is always higher for PM fiber. Second, in the event that some polarization coupling does occur, polarization mode dispersion will be very high. Third, PM fiber is expensive, the cost being dependent on the degree of polarization preservation needed. Thus, PM fiber is impractical for system-wide deployment.
In another approach that has been considered, mechanical polarization controllers have been used to mechanically track the polarization over time. Usually, polarization tracking is performed in two stages. First, the state of polarization is measured. Then, the state of polarization of the receiver and the incoming light signal are adjusted to coincide. Mechanical polarization controllers are used in laboratories throughout the world to conduct telecommunications experiments. However, these devices are largely confined to the laboratory. Even under laboratory conditions these devices have several drawbacks. Mechanical polarization controllers are not robust and require constant supervision to ensure that they are in good working order. Even when the device is working properly, the polarization state must be tracked mechanically over time and there is no straight forward way to do this because there is no tap available. This makes direct monitoring of the state of polarization difficult if not impossible.
In yet another approach that has been considered, polarized light splitters have been used to provide polarization sensitive devices with light signals having known polarization states. Polarized light splitters consist of an input beam splitter connected to a resonant structure, which is connected to an output beam splitter. The input beam splitter divides the light signal into parallel and perpendicular components which are then routed into the resonant structure. Light that is at or near the resonant wavelength is rotated by the resonant structure to a known polarization state. However, light that is not at or near the resonant wavelength passes through the resonant structure unchanged. The output beam splitter recombines the components into a light signal having a known polarization state. This light signal is available for use by the polarization sensitive receiver. Unfortunately, resultant light signal is very narrow-band and only a few wavelengths wide because the spectral components of the signal not at or near the resonant wavelength have been filtered out. This method is also expensive.
Thus, a need exists for a wavelength-insensitive polarization splitter/combiner that can be used to split or combine wide-band polarized light signals, without loss of spectral information, in communications systems having polarization-sensitive components. Especially in systems where cost is a major issue, such as in local or metropolitan area networks.